The present invention relates generally to a fuel supply control system for a gas turbine engine for use with an electric power generator and the like, as the driving means thereof. More particularly, the invention relates to a fuel supply control system for a double-axle gas turbine engine having a compressor shaft and a turbine output shaft.
Generally, the double-axle gas turbine engine is used as a driving means for an alternator and so on. In the case of driving the alternator, the gas turbine engine is required to keep the revolution speed of the turbine output shaft constant in order to maintain the frequency of the output power of the alternator constant. For controlling the revolution speed of the turbine output shaft to a constant value, the fuel amount supplied to the gas turbine engine is controlled by controlling the period of opening of a fuel control valve provided in a main fuel passage which connects between a fuel tank and a fuel injection valve. In practice, the fuel control valve is provided with an electrically controlled actuator which is energized to open the valve to permit the fuel flowing to flow therethrough. The actuator of the fuel control valve is feedback controlled based on a difference between the actual revolution speed of the turbine output shaft and a target speed thereof. For feedback control of the actuator, there is provided a fuel control system in which a pulse width of a pulse signal to be applied to the actuator determines a ratio of an energized period and a deenergized period of the actuator, and is determined corresponding to the difference between the actual revolution speed and the target speed in order to reduce the difference therebetween to zero and to maintain the revolution speed of the turbine output shaft constant.
When a load is applied to the alternator and therefore a load applied to the turbine output shaft is rapidly increased, the revolution speed of the turbine output shaft is decreased. Therefore, the difference between the actual revolution speed and the target speed is increased. Responsive to this, the pulse width of the pulse signal to be applied to the fuel control valve is varied to increase the ratio of the energized period of the actuator to increase the fuel amount supplied to the gas turbine engine. However, the response of the fuel control valve lags with regard to variations of the pulse width of the pulse signal due to its mechanical delay of response. In this respect, the revolution speed of the turbine output shaft is temporarily lowered to lower the frequency of the output power of the alternator. Additionally, since the inertia moment applied to the double-axle gas turbine engine is smaller than that applied to a single-axle gas turbine engine, the ratio of lowering the revolution speed of the turbine output shaft of the double-axle gas turbine engine is relatively larger than that of the single-axle turbine engine.
In the conventional control system, the load applied to the turbine output shaft corresponding to the generator load, is measured. The control system determines the target revolution speed of the turbine output shaft based on the measured load condition of the turbine output shaft. In the low load condition, the target revolution speed of the turbine output shaft is determined at a relatively higher level within a range not exceeding an allowable range of revolution speed of the turbine output shaft. Thus, even when the load applied to the turbine output shaft is increased and thereby the revolution speed is lowered, the turbine output shaft can be maintained in the allowable range of the revolution speed.
However, for determining the revolution speed of the turbine output shaft, a relatively complicated and large or bulky device is necessary. Therefore, the total system of the turbine control system becomes complicated and large.